1. Field of the Invention
The present invention relates to a solar cell and a production process therefor, and more particularly to a solar cell with good energy conversion efficiency and a production process therefor.
2. Related Background Art
In recent years solar cells which convert solar radiation or illumination light into electric energy have been utilized as an energy source in various devices.
Generally, the solar cells have a pn junction or pin junction in a functional part constructed of a semiconductor, and silicon is commonly used as the semiconductor for forming the pn junction (or pin junction). The use of single-crystal silicon is preferred from the aspect of efficiency of converging optical energy into electromotive force, but amorphous silicon is advantageous from the aspects of area increase and cost reduction.
In recent years, the use of polycrystalline silicon has been studied for the purpose of achieving cost as low as amorphous silicon and energy conversion efficiency as high as single-crystal silicon. It was, however, difficult for the conventionally suggested production processes to realize a thickness of polycrystalline silicon below 0.3 mm, because wafers obtained by slicing bulk polycrystalline silicon were used. Thus, the thickness of a semiconductor region was more than necessary for sufficiently absorbing the light, i.e. they were not thin enough with respect to effective use of material. Further, there is a desire to decrease the thickness so as to decrease the cost as well.
Therefore, attempts have been made to form a thin film of polycrystalline silicon, using thin-film technology such as the chemical vapor deposition process (CVD), but at present the crystal size is at most only a few hundredths of a micron, and the energy conversion efficiency is lower than with the wafers sliced from bulk polycrystalline silicon.
Another attempt was made to increase the crystal size by irradiating a polycrystalline silicon thin film with laser light to effect fusion and recrystallization thereof, but the degree of cost reduction is not yet enough and stable fabrication is difficult.
In addition, there is a proposal of a method for forming a crystalline silicon film in a thickness necessary and sufficient to absorb the solar radiation, on a low-cost substrate by radiant heating [Morikawa, Matsuno, Itagaki, Sasaki, and Kumabe; Extended Abstracts for Academic Lecture 18a-SK-11, p. 672 (The 53rd Autumn Meeting, 1992); The Japan Society of Applied Physics].
The above-described method, however, employed metallurgical-grade silicon as a substrate and the size of the substrate was as large as a silicon wafer, thus continuous growth of a large-area silicon layer thereon was not possible.
It is also conceivable to use a metal such as SUS (stainless steel) as a low-cost substrate for growing a large-area silicon layer thereon; this method, however, includes a problem of mixture of unnecessary impurity components into the silicon film and a problem that a polycrystalline silicon film cannot be formed directly on the stainless steel substrate because of a difference of thermal expansion coefficients between them.